At present, our understanding of the pathogenesis of AIDS-related neurological damage is incomplete. However, monocyte derived cells, included macrophages and microglia, appear to play a critical role in the genesis of this condition. Monocytic infiltration of the central nervous system (CNS) is a cardinal feature of AIDS-related neuropathology as well as a significant correlate of dementia. Monocyte-derived cells are the prime targets for HIV-1 in the CNS. Moreover, such cells have been shown to release a number of substances, which are highly toxic to neurons. These substances include the viral gene products Tat, glycoprotein 41 (gp41) and glycoprotein 120 (gp120), as well as the cellular products tumor necrosis factor-xcex1, nitric oxide, platelet activating factor and quinolinate. In vivo studies have shown that many of these toxins are produced predominantly, if not exclusively, by monocyte-derived cells. Also, one study which examined brain tissues from pediatric patients has demonstrated that apoptotic neurons are frequently located in proximity to HIV-1-infected macrophages and microglia.
The present invention makes use of the observation that the HIV-1 Tat protein stimulates the production of monocyte chemoattractant, particularly monocyte chemoattractant protein-1 (MCP-1). MCP-1 is known to be the most potent of a variety of monocyte chemoattractants including RANTES, macrophage inflammatory protein-1xcex1 ((MIP-1xcex1), MIP-1xcex2, MCP-2 and MCP-3. MCP-1 is inducible in astrocytes, which are the most numerous cells in the brain.
The HIV-1 encoded transactivator Tat, a soluble protein which is released from HIV-1 infected cells, can increase NF-xcexaB binding in astrocytes. We have found that Tat affects astrocytic expression and release of this MCP-1 in a dose dependent manner. MCP-1 is elevated in the CNS of patients with AIDS dementia. AIDS dementia is characterized not only by monocytic infiltration of the brain, but by an increase in Tat encoding transcripts.
Human astrocytes produce MCP-1, and this production is increased by the HIV-1 protein Tat. The production of MCP-1 is increased in the brains of AIDS patients with dementia.
Unlike RANTES, MIP-1xcex1 or MIP-1xcex2, MCP-1 does not have significant neutralizing activity against primary viral isolates nor does it inhibit HIV-1 infection of microglia. In addition, while MCP-1 may have some antiviral activity under select in vitro conditions in some experiments it has been associated with an increase in HIV-1 replication. Also, like other xcex2-chemokines, MCP-1 stimulation of select cell types has been associated with increased expression of proinflammatory substances such as interleukin-1xcex2, interleukin-6 and arachidonate. Moreover, MCP-1 stimulation of monocytes has been associated with an increase in the release of superoxide.
MCP-1 can contribute to the monocytic infiltration that has been observed to correlate with HIVD. Monocytic infiltration in turn can be associated with an increase in the release of neurotoxins. Further, it is likely that a positive feedback loop exists whereby more cells in the brain could be infected, leading to increased levels of Tat and hence more MCP-1.
In transgenic mice, glial-specific expression of MCP-1 is associated with pronounced monocytic infiltration of the CNS. MCP-1 injection into the murine hippocampus also leads to the selective recruitment of monocytes. Additionally, monocyte chemoattractant activity in the CSF of patients with viral meningitis can be inhibited with antibodies to MCP-1.
The concentrations of MCP-1 in the CSF of patients with HIV are sufficient to induce monocyte chemotaxis, and it is possible that local amounts in brain tissue are even higher. Therefore, MCP-1 can play a significant role in the pathophysiology of AIDS dementia. Other chemokines, however, may also be present in association with this condition. For example, brain-derived cells can produce MIP-1xcex1, MIP-1xcex2 and RANTES in vitro. Transcripts of MIP-1xcex1, MIP-1xcex2 and RANTES have been detected in HIVD brain tissues through the use of PCR techniques. However, it is not clear as to whether these chemokines are produced in sufficient quantities to induce chemotaxis.
The HIV-1 Tat protein appears to upregulate the expression of MCP-1 in the CNS of patients with HIVD. Tat is essential for viral replication and, in comparison to HIV-1 structural proteins, is a relatively small diffusible molecule. Once released from infected cells, Tat increases MCP-1 expression, though the exact mechanism is not known. One possible mechanism is that its ability to increase NF-xcexaB binding, directly affects MCP-1 expression. At later time points, Tat might also increase MCP-1 expression through indirect mechanisms. For example, Tat could stimulate the production of cytokines that can also induce MCP-1 expression.
The applicant has found that low nanomolar concentrations of Tat were sufficient to increase astrocytic MCP-1 release. These concentrations are slightly lower than those required for gp41 increasing nitric oxide production in mixed neuroglial cultures. Due to the rapid degradation of extracellular protein in autopsy material, as well as cross reactivity of antisera to Tat with endogenous brain proteins, it is difficult to quantitate Tat protein in vivo. However, it has been shown that tat transcripts are elevated in the CNS or AIDS patients with both dementia and encephalitis.
While astrocytes may not be the only CNS cells to produce MCP-1, nor Tat the only stimulus for such production, the ability of Tat to increase astrocytic expression of this chemokine is significant. Not only are astrocytes the most abundant cells in the brain, they are in intimate contact with the blood brain barrier. Such astrocytes could be expected to play an important role in the recruitment of monocytes to the CNS.
The present invention employs the finding that MCP-1 is significantly elevated in the CNS of patients with AIDS dementia in the provision of improved therapies making use of clinically effective inhibitors of MCP-1 and/or its principal receptor CCR-2. These compounds are usefully employed to inhibit the attraction of peripheral monocytes to the CNS even in the presence of stimuli that increase MCP-1 expression. Further, these compounds may be usefully employed to inhibit the activation of monocyte derived cells not only in the CNS but also in the periphery. This invention employs MCP-1 antagonists from other medical fields, such as antagonists used to treat inflammatory conditions such rheumatoid arthritis.